The removal of volatile components from a flowable material, referred to as “devolatilization,” is a necessary step in several industrial processes, including the commercial manufacture of many polymers. In particular, where a polymer is produced from a solution of monomers, it is necessary to remove the solvent and unreacted monomers from the final product. For example, residual monomer and volatiles must be removed from the polymer product in the bulk or solution polymerization of polystyrene, styrene/acrylonitrile copolymers (SAN), or rubber-modified styrene/acrylonitrile copolymers (ABS, AES, etc.), and olefin-based polymers (such as polypropylene, polyethylene, olefin block copolymers, and EPDM).
The separation of the volatile components from a molten polymer solution is generally achieved by evaporation, the process consisting of heating the polymer solution at a temperature higher than the boiling point of the volatile components and removing the evolved volatile components. One method of devolatilization involves passing the polymer solution through a heat exchanger and then into a zone of reduced pressure. Suitable heat exchangers for this purpose, such as those referred to as flat plate heaters or flat plate heat exchangers, comprise a multiplicity of heated plates arranged in stacks or layers, with various heating channels connecting the interior (into which a polymer solution is supplied) and exterior portions of the heater for passage of the solution to be heated and devolatilized. Improved performance is attained by placing the heater within a closed shell which is partially evacuated.
Previous designs of flat plate heaters have been disclosed in U.S. Pat. Nos. 3,014,702; 4,153,501; 4,421,162; 4,423,767; 4,564,063; 4,808,262; 5,084,134; 5,453,158; and 5,861,474, and also in PCT publication WO96/21836.
In order to compete in the global economy, it has become necessary to install polymerization plants having larger capacities (in some cases exceeding 330,000 metric tons per annum, 330 KTA). In plants of that size, even the most efficient conventional designs (such as those disclosed in U.S. Pat. No. 5,453,158) typically reach or exceed the physical size limitations of the flat plate heaters which can be constructed economically and operated successfully, as a practical matter. In particular, those conventional designs are inadequate to achieve the both the necessary degree of devolatilization and the high throughputs of these larger plants, and therefore those heat exchangers have become the capacity-limiting component in the design of devolatilization apparatus for larger polymerization plants.
Thus a need exists for an improved devolatilization apparatus incorporating a plate heat exchanger having an improved heating channel design, which design would allow higher efficiency and throughput, while achieving sufficiently low residual volatiles in the devolatilized material.